Optical frequency division multiplexing transmission system

Abstract

An optical frequency division multiplexing transmission system for sending a large volume of information by a single optical fiber is disclosed. Each of an optical frequency division multiplexing transmitter equipment and an optical frequency division multiplexing receiver equipment has an optical filter having a periodic optical transmission characteristic therein. Each optical filter is stabilized with an absolutely-stabilized standard optical frequency. Each optical signal frequency is stabilized to a periodic transmission characteristic of the optical filter arranged in the optical frequency division multiplexing transmitter equipment, and each local frequency to a periodic transmission characteristic of the optical filter arranged in the optical frequency division multiplexing receiver equipment. Even in the case where no optical signal is applied to the optical frequency division multiplexing receiver equipment due to such causes as a fault of the transmission optical source or the breakage of an optical fiber, the local optical source frequency continues to be controlled in stable fashion.

We claim: 
     
       1. An optical frequency division multiplexing transmission system comprising: an optical frequency division multiplexing transmitter equipment for multiplexing by frequency division and outputting a plurality of optical signals of different wavelengths and an optical frequency division multiplexing receiver equipment including a plurality of heterodyne optical receivers for receiving said optical signals, respectively;   wherein said optical frequency division multiplexing transmitter equipment includes a first optical filter having a periodic optical transmission characteristic, with central frequencies on the frequency axis, first optical filter stabilization means for stabilizing the transmission characteristic of said first optical filter, and an optical signal frequency stabilization circuit for stabilizing the central frequencies of said optical signals, respectively, according to the transmission characteristic of said first optical filter; and   said optical frequency division multiplexing receiver equipment includes a second optical filter having a periodic optical transmission characteristic, with central frequencies, on the frequency axis, second optical filter stabilization means for stabilizing the transmission characteristic of the second optical filter, and a local optical source frequency stabilization circuit for stabilizing the frequencies of local optical sources of the heterodyne optical receivers, respectively, in accordance with the transmission characteristic of the second optical filter;   said system further comprising means for causing the difference between the stabilized central frequencies of the optical signals and the stabilized frequency of the local optical source corresponding to each optical signal to coincide with the optimum intermediate frequency for each heterodyne optical receiver.   
     
     
       2. An optical frequency division multiplexing transmission system according to claim 1, wherein a period of the transmission characteristic of said first optical filter is set to the interval of the central frequencies of said optical signals divided by a natural number. 
     
     
       3. An optical frequency division multiplexing transmission system according to claim 1, wherein a period of the transmission characteristic of the second optical filter is set to the interval of the central frequencies of said optical signals divided by a natural number. 
     
     
       4. An optical frequency division multiplexing transmission system according to claim 3, wherein said period of the transmission characteristic of the second optical filter is set to said intermediate frequency divided by a natural number. 
     
     
       5. An optical frequency division multiplexing transmission system according to claim 1, wherein said first optical filter stabilization means includes: a first optical source with an optical standard frequency for outputting an absolutely-stabilized optical standard frequency, an optical frequency modulator for frequency-modulating the light outputted from said optical source with a frequency f 0  ;   an optical coupler for multiplexing the frequency-modulated light and said optical frequency division multiplexing signal and inputting a multiplexed signal to said first optical filter;   a photo-diode for converting the light outputted from said optical filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component proximate to frequency f 0  from the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying the [extracted]signal extracted from the bandpass filter with said sinusoidal wave signal;   a low-pass filter, with cut-off frequency of f LPF   f 0 , for extracting an error signal from the multiplied signal; and   a temperature control circuit for controlling a temperature of the first optical filter based on the error signal.   
     
     
       6. An optical frequency division multiplexing transmission system according to claim 5, wherein said second optical filter stabilization means includes: a second optical source with the optical standard frequency for outputting an absolutely-stabilized optical standard frequency;   an optical frequency modulator for frequency modulating the light outputted from the optical source with frequency f 0  ;   a star coupler for multiplexing said local optical signals;   an optical coupler for multiplexing the frequency-modulated light and the multiplexed local optical signals and inputting the resulting signal to the second optical filter;   a photo-diode for converting the light outputted from said filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component of frequency f 0  from the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying the signal extracted from the bandpass filter with said sinusoidal wave signal to produce a multiplied signal;   a low-pass filter, with a cut-off frequency of f LPF   f 0 , for extracting an error signal from the multiplied signal; and   a temperature control circuit for controlling a temperature of the second optical falter based on the error signal;   and the optical frequency of said first optical source with the optical standard frequency is caused to coincide with the optical frequency of said second optical source with the optical standard frequency.   
     
     
       7. An optical frequency division multiplexing transmission system according to claim 6, wherein a period of the transmission characteristic of said first optical filter is set to the interval of the central frequencies of said optical signals divided by a natural number, a period of the transmission characteristic of said second optical filter is set to the interval of the central frequencies of said optical signals divided by a natural number, and the period of the transmission characteristic of the second optical filter is set to the intermediate frequency divided by a natural number.   
     
     
       8. An optical frequency division multiplexing transmission system according to claim 7, wherein the optical frequency of said first and second optical sources with the optical standard frequency, a frequency associated with selected one of maximum and minimum transmission characteristics of said first optical filter and a frequency associated with selected one of maximum and minimum transmission characteristics of said second optical filter, are caused to coincide with each other. 
     
     
       9. An optical frequency division multiplexing transmission system according to claim 7, wherein the central frequencies of the optical signals are stabilized to an optical frequency associated with selected one of maximum and minimum transmission characteristics of said first optical filter by said optical signal frequency stabilization circuit, and the frequencies of the local optical sources of the heterodyne optical receivers are stabilized to a frequency associated with selected one of maximum and minimum transmission characteristics of said second optical filter by the local optical source frequency stabilization circuit. 
     
     
       10. An optical frequency division multiplexing transmission system according to claim 7, wherein: the optical frequency of said first and second optical sources with the optical standard frequency, a frequency associated with selected one of maximum and minimum transmission characteristics of said first optical filter and a frequency associated with selected one of maximum and minimum transmission characteristics of the second optical filter are caused to coincide with each other;   the central frequencies of the optical signals are stabilized to different optical frequencies respectively associated with selected one of maximum and minimum transmission characteristics of said first optical filter by said optical signal frequency stabilization circuit; and   the frequencies of the local optical sources of said heterodyne optical receivers are stabilized to a frequency associated with selected one of maximum and minimum transmission characteristics of said second optical filter by said local optical source frequency stabilization circuit.   
     
     
       11. An optical frequency division multiplexing transmission system according to claim 5, wherein said optical signal frequency stabilization circuit includes a multiplier for multiplying an information signal applied to the optical frequency division multiplexing transmitter equipment with said detected signal to produce a multiplied signal, and a low-pass filter for extracting an error signal from the multiplied signal. 
     
     
       12. An optical frequency division multiplexing transmission system according to claim 1, wherein said second optical filter stabilization means includes: a second optical source with an optical standard frequency for outputting an absolutely-stabilized optical standard frequency;   an optical frequency modulator for frequency modulating the light outputted from the second optical source with a frequency f 0  ;   a star coupler for multiplexing said local optical signals;   an optical coupler for multiplexing the frequency-modulated light and the multiplexed local optical signals and inputting a multiplexed signal to the second optical filter;   a photo-diode for convening the light outputted from said optical filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component of frequency f 0  from the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying the signal extracted from the band pass filter and the sinusoidal wave signal to produce a multiplied signal;   a low-pass filter, with a cut-off frequency of f LPF   f 0 , for extracting an error signal from the multiplied signal; and   a temperature control circuit for controlling a temperature of the second optical filter based on the error signal.   
     
     
       13. An optical frequency division multiplexing transmission system according to claim 12, wherein said local optical source frequency stabilization circuit includes a bandpass filter for extracting the components of a frequency f 1  from said detected signal, an oscillator, with frequency f 1 , for outputting a sinusoidal wave signal, a multiplier for multiplying the extracted signal with the sinusoidal wave signal, a lowpass filter, with a cut-off frequency f LPF   f 1 , for extracting an error signal from the multiplied signal, and an adder for adding the error signal and a bias signal for the sinusoidal wave signal to each other. 
     
     
       14. An optical frequency division multiplexing transmission system according to claim 13, wherein the frequencies of the oscillators in said local optical source frequency stabilization circuit of said heterodyne optical receiver are different from each other. 
     
     
       15. An optical frequency division multiplexing transmission system according to claim 1, wherein the first optical filter is a Fabry-Perot optical resonator and the second optical filter is a Fabry-Perot optical resonator. 
     
     
       16. An optical frequency division multiplexing transmission system according to claim 1, wherein the first optical filter is a Mach-Zehnder optical filter and the second optical filter is a Mach-Zehnder optical filter. 
     
     
       17. An optical frequency division multiplexing transmission system according to claim 1, wherein said heterodyne optical receiver includes a local optical source frequency stabilization circuit, a local optical source, an optical coupler for dividing a local optical signal from the local optical source, a polarization diversity optical receiver for receiving an optical signal from the optical coupler and outputting a base band signal based on one of the outputs of said optical coupler, and a decision and regeneration circuit for discriminating and regenerating an information signal based on the base band signal output from the polarization diversity optical receiver. 
     
     
       18. An optical frequency division multiplexing transmission system according to claim 17, wherein said polarization diversity optical receiver includes a polarization isolator for isolating an optical signal into horizontal and vertical polarization optical signals, an optical coupler for dividing said local optical signal into first and second local optical signals, a first balanced receiver for subjecting a horizontal polarization optical signal to heterodyne detection by use of the first local optical signal, a first demodulator circuit for converting a first intermediate frequency signal outputted from the first balanced receiver into a first base band signal, a second balanced receiver for subjecting a vertical polarization optical signal to heterodyne detection by use of a second local optical signal, a second demodulator circuit for converting a second intermediate frequency signal outputted from the second balanced receiver into a second base band signal, and an adder for adding the first and second base band signals to each other and outputting the base band signal. 
     
     
       19. An optical frequency division multiplexing transmission system according to claim 1, wherein the optical frequency division multiplexing transmitter equipment and the optical frequency division multiplexing receiver equipment are connected by an optical fiber. 
     
     
       20. An optical frequency division multiplexing transmission system according to claim 19, wherein an optical amplifier is connected to said optical fiber. 
     
     
       21. An optical frequency division multiplexing transmission system according to claim 1, wherein said first optical filter stabilization means includes: a first optical source with the optical standard frequency for outputting light frequency modulated with frequency f 0  around an absolutely stabilized optical standard frequency;   an optical coupler for multiplexing the light outputted from said first optical source with said optical frequency division multiplexing signal and inputting the resulting signal to said first optical filter;   a photo-diode for converting the light outputted from said optical filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component proximate to frequency f 0  frown the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying the signal extracted from the bandpass filter and said sinusoidal wave signal with each other to produce a multiplied signal;   a low-pass filter, with a cut-off frequency f LPF   f 0 , for extracting an error signal from the multiplied signal; and   a temperature control circuit for controlling a temperature of said first optical filter based on the error signal.   
     
     
       22. An optical frequency division multiplexing transmission system according to claim 1, wherein said second optical filter stabilization means includes: a second optical source with the optical standard frequency for outputting light frequency modulated with a frequency f 0  around an absolutely stabilized optical standard frequency;   a star coupler for multiplexing the local optical signals;   an optical coupler for multiplexing the frequency-modulated light and said multiplexed local optical signals and inputting the resulting signal to said second optical filter;   a photo-diode for convening the light outputted from said optical filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component of frequency f 0  from the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying the signal extracted from the bandpass filter with said sinusoidal wave signal to produce a multiplied signal;   a low-pass filter, with a cut-off frequency f LPF   f 0  for extracting an error signal from the multiplied signal; and   a temperature control circuit for controlling a temperature of the second optical filter based on the error signal.   
     
     
       23. An optical frequency division multiplexing transmission system according to claim 1, comprising a plurality of optical frequency division multiplexing means. 
     
     
       24. An optical frequency division multiplexing transmission system according to claim 23, wherein a plurality of optical frequency division multiplexing transmission means are connected in the last half stage of the optical frequency division multiplexing transmission means, and a plurality of optical frequency division multiplexing transmission means are connected in the last half stage of the optical frequency division multiplexing transmission means. 
     
     
       25. An optical frequency division multiplexing transmission system according to claim 23, wherein the optical frequency division multiplexing transmitter equipment and the optical frequency division multiplexing receiver equipment other than the optical frequency division multiplexing transmitter equipment at the sending end and the optical frequency division multiplexing receiver equipment at the receiving end are replaced, in part or in whole, by optical amplifiers. 
     
     
       26. An optical frequency division multiplexing transmission system according to claim 1, wherein optical frequency division multiplexing means for transmitting a natural number m out of N channels is connected with optical frequency division multiplexing means for transmitting the remainder, N-m, of the N channels in the last half stage of the optical frequency division multiplexing transmission system for transmitting information signals of the number N of channels. 
     
     
       27. An optical frequency division multiplexing transmission system according to claim 1, wherein optical frequency division multiplexing transmission means capable of transmitting m+p channels at the same time is connected in the last half stage of said optical frequency division multiplexing means for transmitting m-channel, where m is a natural number, information signals and the optical frequency division multiplexing transmission means for transmitting p-channel, where p is a natural number, information signals. 
     
     
       28. An optical frequency division multiplexing transmission system according to claim 1, wherein at least one of the optical sources for generating a plurality of optical signals is arranged at a place different from said first optical filter stabilization means. 
     
     
       29. An optical frequency division multiplexing transmission system according to claim 28, wherein the information for stabilizing the central frequency of said optical source is transmitted from the first optical filter stabilization means to said optical source by use of an optical fiber. 
     
     
       30. An optical frequency division multiplexing transmission system comprising an optical frequency division multiplexing transmitter equipment for multiplexing by frequency division and outputting a plurality of optical signals having different wavelengths, and an optical frequency division multiplexing receiver equipment having a tunable heterodyne optical receiver for selectively receiving one of said optical signals, wherein said optical frequency division multiplexing transmitter equipment includes: a first optical filter having a periodic optical transmission characteristic, with central frequencies, on the frequency axis, a first optical filter stabilization means for stabilizing the transmission characteristic of the first optical filter, and an optical signal frequency stabilization circuit for stabilizing the central frequencies of said optical signals on the basis of the transmission characteristic of the first optical filter;   said optical frequency division multiplexing receiver equipment includes a second optical filter having a periodic optical transmission characteristic on the frequency axis, a second optical filter stabilization means for stabilizing the transmission characteristic of the second optical filter, a local optical source frequency stabilization circuit for stabilizing the frequency of a tunable local optical source of the heterodyne optical receiver on the basis of the transmission characteristic of the second optical filter, and an optical tuner for tuning the frequency of said local optical source; and   the difference between the stabilized central frequencies of said optical signals and the frequency of the local optical source at the time of receiving each optical signal is caused to coincide substantially with the optimum intermediate frequency for the heterodyne optical receiver.   
     
     
       31. An optical frequency division multiplexing transmission system according to claim 30, wherein said first optical filter stabilization means includes: a first optical source with an optical standard frequency for outputting an absolutely stabilized optical standard frequency;   an optical frequency modulator for frequency modulating the light outputted from said optical source with frequency f 0  ;   an optical coupler for multiplexing a frequency-modulated light and said optical frequency division multiplexing signal with each other and inputting the resulting signal to said first optical filter;   a photo-diode for converting the light outputted from said optical filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component proximate to frequency f 0  from the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying said signal extracted from said bandpass filter with said sinusoidal wave signal to produce a multiplied signal;   a low-pass filter, with a cut-off frequency f LPF   f 0 , for extracting an error signal from the multiplied signal; and   a temperature control circuit for controlling a temperature of said first optical filter based on the error signal.   
     
     
       32. An optical frequency division multiplexing transmission system according to claim 31, wherein: said second optical filter stabilization means includes a second optical source with an optical standard frequency for outputting an absolutely stabilized optical standard frequency, an optical frequency modulator for frequency-modulating the light outputted from an optical source with frequency f 0 , an optical coupler for multiplexing a frequency-modulated light with said local optical signal and inputting the resulting signal to said second optical filter, a photo-diode for converting the light outputted from said optical filter into an electrical signal and producing a detected signal, a bandpass filter for extracting a signal component of frequency f 0  from the detected signal, an oscillator for outputting a sinusoidal wave signal of frequency f 0 , a multiplier for multiplying the signal extracted from the bandpass filter with the sinusoidal wave signal to produce a multiplied signal, a lowpass filter, with a cut-off frequency f LPF   f 0 , for extracting an error signal from the multiplied signal, and a temperature control circuit for controlling a temperature of the second optical filter based on the error signal; and   the optical frequency of said first optical source with the optical standard frequency is caused to coincide with that of said second optical source with the optical standard frequency, a period of the transmission characteristic of said first optical filter is set to the interval of the central frequencies of said optical signals divided by a natural number, a period of the transmission characteristic of said second optical filter is set to the interval of the central frequencies of said optical signals divided by a natural number, and the period of the transmission characteristic of the second optical filter is set to an intermediate frequency divided by a natural number.   
     
     
       33. An optical frequency division multiplexing transmission system according to claim 32, wherein: the optical frequency of said first and second optical sources with the optical standard frequency, a frequency associated with selected one of maximum and minimum transmission characteristics of said first optical filter, and the frequency associated with selected one of maximum and minimum transmission characteristics of said second optical filter are caused to coincide with each other;   the central frequencies of said optical signals are stabilized to an optical frequency associated with selected one of maximum and minimum transmission characteristic of said first optical filter by said optical signal frequency stabilization circuit; and   the frequency of the local optical source of said heterodyne optical receiver is set to a frequency associated with selected one of maximum and minimum transmission characteristics of said second optical filter by said local optical source frequency stabilization circuit.   
     
     
       34. An optical frequency division multiplexing transmission system according to claim 30, wherein said second optical filter stabilization means includes: a second optical source with the optical standard frequency for outputting an absolutely stabilized optical standard frequency;   an optical frequency modulator for frequency modulating the light outputted from an optical source with frequency f 0  ;   an optical coupler for multiplexing a frequency-modulated light and said local optical signal with each other and inputting the resulting signal to said second optical filter;   a photo-diode for converting the light outputted from said optical filter into an electrical signal and producing a detected signal;   a bandpass filter for extracting a signal component of frequency f 0  from the detected signal;   an oscillator for outputting a sinusoidal wave signal of frequency f 0  ;   a multiplier for multiplying the signal extracted from the bandpass filter with said sinusoidal wave signal to produce a multiplied signal;   a low-pass filter, with a cut-off frequency f LPF   f 0 , for extracting an error signal frown the multiplied signal; and   a temperature control circuit for controlling a temperature of the second optical filter based on the error signal.   
     
     
       35. An optical frequency division multiplexing transmission system according to claim 34, wherein said local optical source frequency stabilization circuit includes a bandpass filter for extracting a component of frequency f 1  from said detected signal, an oscillator, with frequency f 1 , for outputting a sinusoidal wave signal, a multiplier for multiplying an extracted signal with a sinusoidal wave signal to produce a multiplied signal, a low-pass filter, with a cut-off frequency f LPF   f 1 , for extracting an error signal from the multiplied signal, and an adder for adding the error signal and the sinusoidal wave signal bias to each other. 
     
     
       36. An optical frequency division multiplexing transmission system according to claim 30, wherein said heterodyne optical receiver includes a local optical source frequency stabilization circuit, a tunable local optical source, an optical coupler for dividing a local optical signal, a polarization diversity optical receiver for receiving an optical signal and outputting a base band signal by use of one of the outputs of said optical coupler, and a decision and regeneration circuit for discriminating and regenerating an information signal from the signal outputted from the polarization diversity optical receiver. 
     
     
       37. An optical frequency division multiplexing transmission system according to claim 30, wherein said local optical source frequency stabilization circuit has built therein means for switching the local optical source frequency stabilization circuit to and from the tunable local optical source. 
     
     
       38. An optical frequency division multiplexing transmission system according to claim 37, wherein said optical tuner includes a low-pass filter for removing a signal component of frequency f 1 , a pulse counter for counting pulses generated as a detected signal at the time of switching the local frequencies, a comparator for comparing the number of the counted pulses with a number of pulses to be generated at the time of switching channels, and a controller for controlling said switching means and the frequency of said tunable local optical source in accordance with the output of said comparator. 
     
     
       39. An optical frequency division multiplexing transmission system according to claim 38, wherein said heterodyne optical receiver includes a local optical source frequency stabilization circuit, an optical tuner, a tunable local optical source, an optical coupler for dividing a local optical signal, a polarization diversity optical receiver for receiving an optical signal and outputting a base band signal by use of one of the outputs of said optical coupler, and a decision and regeneration circuit for discriminating and regenerating an information signal from the signal outputted from the polarization diversity optical receiver. 
     
     
       40. An optical frequency division multiplexing transmission system according to claim 30, wherein said optical frequency division multiplexing transmitter equipment and said optical frequency division multiplexing receiver equipment are connected to each other by an optical fiber. 
     
     
       41. An optical frequency division multiplexing transmission system according to claim 40, wherein an optical amplifier is connected to said optical fiber. 
     
     
       42. An optical frequency division multiplexing transmission system according to claim 30, wherein a plurality of optical frequency division multiplexing transmission means described in claim 30 are connected in the last half stage of the optical frequency division multiplexing transmission system. 
     
     
       43. An optical frequency division multiplexing receiver equipment comprising: an optical demultiplexer for isolating optical signals of a number N of channels multiplexed by frequency division;   a number N of heterodyne optical receivers for receiving the isolated optical signals and regenerating an information signal respectively;   an optical multiplexer for multiplexing local optical signals outputted from local optical sources of said heterodyne optical receivers;   an optical filter inputted with said multiplexed light and having a periodic optical transmission characteristic on the frequency axis;   optical filter stabilization means for stabilizing the transmission characteristic of the optical filter; and   a number N of local optical source frequency stabilization circuits for stabilizing the frequencies of the local optical sources respectively on the basis of the transmission characteristic of the optical filter.   
     
     
       44. An optical frequency division multiplexing receiver equipment comprising: a heterodyne optical receiver for selectively receiving one channel from a number N of channels of optical signals multiplexed by frequency division;   an optical coupler for dividing the local optical source signal outputted from a tunable local optical source of the heterodyne optical receiver into two parts;   an optical filter inputted with one of the divided optical signals and having a periodic optical transmission characteristic on the frequency axis;   optical filter stabilization means for stabilizing the transmission characteristic of the optical filter;   a local optical source frequency stabilization circuit for stabilizing the frequency of the local optical source on the basis of the transmission characteristic of the optical filter; and   an optical tuner for tuning the frequency of the local optical source.   
     
     
       45. An optical frequency division multiplexing transmitter equipment comprising: a number N of optical sources for converting information signals of a number N of channels into optical signals respectively:   an optical filter for optically multiplexing the optical signals of N channels and outputting an optical frequency division multiplexing signal, said optical filter having a periodic optical transmission characteristic on the frequency axis;   an optical coupler for dividing the optical frequency division multiplexing signal into two parts;   optical filter stabilization means for stabilizing the transmission characteristic of the optical filter; and   a number N of optical signal frequency stabilization circuits for stabilizing the central frequencies of said N-channel optical signals on the basis of the transmission characteristic of the optical filter.   
     
     
       46. An optical frequency division multiplexing receiver equipment comprising: an optical demultiplexer for isolating a plurality of optical signals of N channels multiplexed by frequency division;   a number N of heterodyne optical receivers for receiving the divided optical signals and regenerating information signals;   an optical filter for multiplexing the local optical signals outputted from local optical sources of said heterodyne optical receivers respectively, and having a periodic optical transmission characteristic on the frequency axis;   optical filter stabilization means for stabilizing the transmission characteristic of the optical filter; and   a number N of local optical source frequency stabilization circuits for stabilizing the frequencies of the local optical sources respectively on the basis of the transmission characteristic of the optical filter.   
     
     
       47. An optical frequency stabilization system comprising: a plurality of optical sources for generating light of specific optical frequencies respectively;   an optical frequency regulator for regulating the optical frequencies of said optical sources;   optical input means including a transmission path for connecting the optical sources and the optical frequency regulator to each other for enabling the optical frequency regulator to receive a part of the optical signals from said optical sources;   means for determining a deviation between the regulated optical frequency defined in said optical frequency regulator and the frequency of said optical signals; and   optical output means for sending out said deviation to said optical sources through said transmission path;   each of said optical sources including means for correcting the optical frequencies of the respective optical sources in such a manner as to maintain said deviation at selected one of zero and a predetermined value.   
     
     
       48. An optical frequency stabilization system according to claim 47, wherein said transmission path is one for optical transmission. 
     
     
       49. An optical frequency stabilization system comprising: a plurality of optical sources for generating light of specific optical frequencies respectively;   an optical frequency regulator for regulating the optical frequencies of said optical sources; and   optical output means including an optical transmission path for connecting said optical sources and said optical frequency regulator to each other for enabling said optical frequency regulator to send out optical signals of regulated optical frequencies defined in said regulator;   each of said optical sources including detector means for detecting the deviation between the optical frequencies thereof and said regulated optical frequency and correcting means for correcting the optical frequencies of said optical sources respectively in such a manner as to maintain said deviation at selected one of zero and a predetermined value.   
     
     
       50. An optical frequency stabilization system according to claim 49, wherein said optical sources are for a plurality of optical transmitter/receivers of an optical communications system for transmitting information by optical frequency multiplexing using an optical signal. 
     
     
       51. An optical frequency stabilization system according to claim 50, wherein the wavelength band of said optical signal used for transmitting said deviation value and that of an optical signal for transmitting the information transmitted/received by said optical transmitter/receivers are different from each other. 
     
     
       52. An optical frequency stabilization system according to claim 49, wherein said optical frequency regulator includes an optical resonator having a periodic transmission characteristic on the optical frequency axis and optical frequency control means having an optical frequency associated with selected one of maximum and minimum intensities of selected one of transmitted and reflected light of said optical resonator as a regulated optical frequency. 
     
     
       53. An optical frequency stabilization system according to claim 52, wherein: said optical transmitter/receiver includes first frequency modulator means for frequency-modulating the light of said optical sources with different frequencies respectively; and   said means for determining the deviation of said optical frequency regulator includes means for applying a part of the optical signals modulated by said first frequency modulator means to said optical frequency control means, a detector for detecting selected one of transmitted and reflected lights from said optical frequency control means and converting said light into an electrical signal, and synchronous detection means for synchronously detecting the output of said detector in time division by the signals of different frequencies.   
     
     
       54. An optical frequency stabilization system according to claim 53, wherein said optical frequency regulator includes means for sending out an optical signal containing said deviations in time division, and each of said optical transmitter/receivers includes means for securing synchronization with said optical frequency regulator and isolating a signal to be received thereby. 
     
     
       55. An optical frequency stabilization system according to claim 53, wherein said optical frequency regulator includes means for sending out an optical signal containing said deviations as selected one of frequency- and intensity-modulated signals, and each of said optical transmitter/receivers includes means for isolating only a signal subjected to selected one of frequency and intensity-modulations with a frequency specific thereto from selected one of the frequency- and intensity-modulated signals, respectively. 
     
     
       56. An optical frequency stabilization system according to claim 53, wherein said optical frequency regulator includes means for sending out optical signals containing a plurality of deviations by adding thereto a header specific to the optical transmitter/receivers respectively, and each of said optical transmitter/receivers includes means for receiving only said signals with a header specific to respective optical transmitter/receivers. 
     
     
       57. An optical frequency stabilization system according to claim 53, wherein said optical frequency regulator includes means for sending out by changing the wavelengths of the optical signals containing a plurality of deviations, and each of said optical transmitter/receivers includes means for isolating only a signal of a wavelength specific to said respective optical transmitter/receivers from said optical signals. 
     
     
       58. An optical frequency stabilization system according to claim 52, wherein: said optical transmitter/receivers include first frequency modulator means for frequency-modulating the light of said optical sources with the same frequency respectively; and   said means for determining the deviation of said optical frequency regulator includes means for applying a part of the optical signals modulated by said frequency modulator means to said optical frequency control means, a detector for detecting selected one of transmitted and reflected light from said optical frequency control means and converting said light into an electrical signal, and synchronous detection means for synchronously detecting the output of said detector in time division with said signals of the same frequency.   
     
     
       59. An optical frequency stabilization system according to claim 52, wherein said optical resonator of said optical frequency regulator includes means connected to a standard optical source with an optical fiber for controlling selected one of maximum and minimum values of said optical resonator with the frequencies of the optical signals outputted from said standard optical source. 
     
     
       60. An optical frequency stabilization system according to claim 49, wherein: said optical frequency regulator and said optical transmitter/receivers include synchronizing means,   said sending means of said optical frequency regulator is configured in such a manner as to transmit said optical signals in time division, and   said synchronizing means of said optical transmitter/receivers is configured in such a manner that only one of the optical transmitter/receivers receives the optical signals of the regulated optical frequency at a time.   
     
     
       61. An optical frequency stabilization system according to claim 49, wherein said sending means of said optical frequency regulator is configured in such a manner as to transmit the optical signals of said regulated optical frequency by intensity-modulation with a frequency specific to each of said optical transmitter/receivers, and said optical transmitter/receivers include means for receiving only the optical signals intensity-modulated with a frequency specific to said optical transmitter/receivers, respectively.